The present invention relates to a method and a device for adjusting the braking action on the wheels of a vehicle, having the features of the preamble of the independent claims.
For transmitting the engine output torque to the drive wheels, so-called equalizing gears, or differentials, are known from the related art. They function primarily to equalize the rpm differences between the driven wheels of one axle, for example during cornering. In this context, a mechanical differential, a so-called transverse differential, is usually assigned to one drive axle.
If more than one axle is driven, for example in four-wheel-drive motor vehicles, then a further differential (center differential) is additionally present, primarily so that rpm differences between the driven axles are equalized.
Mechanical differentials, however, can only to a limited extent equalize the rpm differences between the left and right wheels, or between the rear and front axles. If the rpm differences exceed a critical limiting value, then the differential (and the tire) is destroyed by the excessive centrifugal force. The vehicle can no longer be driven and the result is an expensive repair of the affected parts.
For cost reasons, four-wheel-drive motor vehicles are increasingly equipped with open differentials. By way example, in order to be able to transfer a drive torque in one spinning wheel of an axle to the other wheel of the axle, especially in off-road vehicles, differentials are used that can be mechanically locked. Similarly, the center differential can be designed so as to be lockable, so that even in the case of two spinning wheels on one axle a drive torque can become effective on the other axle.
However, this locking function, in so-called open differentials that are designed so as not to be mechanically lockable, can also be realized by a controlled, one-sided braking engagement (BMR, braking torque control), and in four-wheel-drive motor vehicles also by an axle-based braking engagement. The spinning wheel, or wheels, of one axle can be braked in a controlled, or wheel-specific, manner.
Precisely in four-wheel-drive motor vehicles having open differentials, however, the differentials can be damaged very quickly if three wheels are standing still (e.g., on asphalt) and only one wheel is situated on a sheet of ice (or xe2x80x9cis hanging in the airxe2x80x9d) and the driver has caused the engine to xe2x80x9cracexe2x80x9d in the engaged state. If the one-sided braking engagement does not now provide for a braking of this wheel, then the entire drive torque acts on this one wheel in an accelerating manner. In this context, the engine/wheel speed ratio is twice as high as in a vehicle having a conventional drive. An individual wheel, in this context, can be accelerated within one second to several hundred km/h. This happens especially when faulty gear changing also occurs on the part of the automatic transmission. As a result of these enormous centrifugal forces, the tire can be destroyed or at least suffer structural damage so that later a blowout of the tire can result with fatal consequences.
Even if the differentials used are constructed in a mechanically robust manner and endure very high rpm differences at least for short periods, the stability of the tires is a serious subject. This is so especially in the case of off-road tires, which are in any case not designed for very high driving speeds.
The objective of the present invention lies in assuring an effective protection of differentials and tires, using simple means.
This objective is achieved by the characterizing features of the independent claims.
The present invention in a first variant relates to a method and a device for adjusting the output torque of an internal combustion engine in a motor vehicle having at least four wheels that are arranged on at least two axles and that are driven by the internal combustion engine.
For transmitting the drive, at least one transverse differential is provided that is assigned to an axle having driven wheels, and a center differential, that is assigned to two axles having driven wheels. For the driven case of the vehicle, a deactivatable, wheel-specific control system (BMR) for the braking action on the wheels and/or a control system (AMR) for the output torque of the internal combustion engine is provided. The crux of the present invention lies in the fact that, in a deactivated, wheel-specific control system for the braking action and/or control system for the output torque of the internal combustion engine
rpm quantities are measured which represent the rotational motions of the vehicle wheels, and
as a function of the rpm quantities, difference quantities are generated which represent the rpm and/or rotational speed differences at the outputs of the differentials, and
the difference quantities are compared with specifiable threshold values, and
the output torque of the internal combustion engine is adjusted in the sense of a reduction if at least one threshold value is exceeded.
A second variant of the present invention relates to a method and a device for adjusting the output speed of the internal combustion engine in a motor vehicle having at least four wheels that are arranged on at least two axles and that are driven by the internal combustion engine. In this context as well, at least one transverse differential is provided, which is assigned to an axle having driven wheels, and one center differential that is assigned to two axles having driven wheels. Then, in accordance with the invention,
rpm quantities are measured that represent the rotational motions of the vehicle wheels, and
as a function of the rpm quantities, difference quantities are generated which represent the rpm and/or rotational speed differences at the outputs of the differentials, and
the difference quantities are compared with specifiable threshold values, and
the output speed of the internal combustion engine is limited to a specifiable value if at least one threshold value is exceeded.
The following is presented as background to the present invention:
xe2x80x9cNormally,xe2x80x9d in the driven case, the locking system described above, by an active wheel-specific braking (BMR), prevents excessive rpm differences at the outputs of the differentials. However, there exists a series of possibilities that lead to a deactivation of the braking torque control system (BMR):
In the driven case, the driver intentionally brakes, for example in an off-road setting, or he does so unintentionally (a xe2x80x9ctwo-footedxe2x80x9d driver).
The braking torque control system (BMR) on one wheel can be deactivated due to a supposed overheating of the wheel brake.
Similarly, a faulty braking action on a specific wheel, for example as a result of a defective wheel brake, can lead to the BMR becoming inoperative.
In addition to the braking torque control system (BMR), a drive torque control system (AMR) can also be provided, which adjusts the drive torque effective on the wheels such that no excessive drive slip can arise. The drive torque control system (AMR) can be deactivated, for example by the driver. For instance, this can take place when the driver switches off a driving stability control system.
The wheel-specific control system (BMR) for the braking action on the wheels can therefore be deactivated if
a braking process triggered by the driver is occurring and/or
the measured temperature of at least one wheel brake unit exceeds a specifiable threshold value and/or
the driver manually switches off the control system.
According to the present invention, the problem described above is solved by the rpm differences being calculated and monitored through evaluating the wheel speeds. If a limiting value is exceeded, then the engine torque, or the engine speed, is reduced or limited by a regulator, so that no damage can occur to the vehicle. In this way, it is also possible to effectively deal with faulty operation on the part of the driver.
In one advantageous refinement of the present invention in the first variant, it is provided that when at least one threshold value is established as having been exceeded, the above-mentioned control system (AMR) for the output torque of the internal combustion engine is activated, the system acting in the driven case. As a result, it is assured that no excessive drive slip arises on the wheels.
In one advantageous embodiment of the present invention in the second variant, it is provided that the limiting value, to which the engine output speed is limited, is stipulated as a function of the differential whose difference quantity exceeds the specifiable threshold value.
Generally speaking, a manual or automatic transmission is arranged downstream of the internal combustion engine. In the second variant of the present invention, it can be provided that the limiting value, to which the engine output speed is limited, is stipulated as a function of the currently set speed ratio of the transmission.
Further advantageous embodiments can be found in the subclaims.